There is no such a thing as unlimited growth, absolutely. 21000 will not happen. You hit some sort of resource limitation before that point. This is just a hypothetical situation where the resource that is limited is not energy or matter.

Back when the Zero Point Energy craze started, it seemed like all this "free energy" would be coming at the cost of the expansion of the universe. So it seemed like if you used enough of this "free energy", you'd help bring about the collapse of the universe.

As for whether EMdrive, Woodward Drive, etc could become "free energy" generators, likewise it seems unlikely. To me, if these drives work, it seems like they could only generate energy by stealing momentum/kinetic-energy and converting it into electricity/etc - in other words, the exact inverse of what the drives are alleged to do.

... so I predict some fraction of the civilisation that devotes all it's effort to speed, and is always ahead of the destruction. Maybe it also keeps leaving boobytraps to slow down the expanding horde behind it, or any other young race that stumbles upon this technology.

How did a discussion on pushing against the universe turn into creating black holes? Given that the expansion of the universe seems to be increasing, I don't see much worry about causing the collapse of the universe. Seems to be plenty of dark energy to go around.

I mean, theoretically fusion would consume all mater in the universe, but that doesn't seem to be happening.

Here's an interesting concept. The universe is getting lighter, right? Every star is fusing matter into energy. Every black hole is converting mass into energy.How much matter does the universe convert into energy every second? How much is gravitational attraction reduced every second as all that energy spreads out? Is that driving expansion?

And why would an extraction of that energy automatically come from the kinetic energy of matter, a very small fraction of the whole?

Here's an interesting concept. The universe is getting lighter, right? Every star is fusing matter into energy. Every black hole is converting mass into energy.How much matter does the universe convert into energy every second? How much is gravitational attraction reduced every second as all that energy spreads out? Is that driving expansion?

And why would an extraction of that energy automatically come from the kinetic energy of matter, a very small fraction of the whole?

it's not getting lighter though. at least not due to fusion. IDK what would be making it lighter or if anything is. However; energy is neither created nor destroyed. The energy from fusion has a mass equivalent. That mass equivalent is still subject the gravitational forces. The weight of the universe is still the same.

If anything, we have seen that humans are tending to breed less and less. Even in some third world countries (like here in Brazil), fertility rates are below 2 children by woman already.

In an advanced interstellar society, I guess couples would wait 100 years before having children...

I am not sure that is accurate measure. Mainly because I think the reason fertility rates are so low in advanced countries is because raising kids is not easy. If the technology this enables drastically reduces the cost both financial and otherwise to raising kids. I can easily see more people having kids on average as a result. Especially if access to that technology is readily available to everyone in the civilization.

Raising kids must be pretty easy in countries like Mali and Niger. Not unexpected, seeing as they are the most technologically advanced countries in the world.

Here's an interesting concept. The universe is getting lighter, right? Every star is fusing matter into energy. Every black hole is converting mass into energy.How much matter does the universe convert into energy every second? How much is gravitational attraction reduced every second as all that energy spreads out? Is that driving expansion?

And why would an extraction of that energy automatically come from the kinetic energy of matter, a very small fraction of the whole?

it's not getting lighter though. at least not due to fusion. IDK what would be making it lighter or if anything is. However; energy is neither created nor destroyed. The energy from fusion has a mass equivalent. That mass equivalent is still subject the gravitational forces. The weight of the universe is still the same.

What I was discussing is the expansion of the universe. Matter doesn't expand. Instead it pulls other matter in, creates gravity wells and warps space, trapping more matter. But energy expands outward. Photons and neutrinos (equivalent since they don't react much with matter) go flying out of gravity wells. In that way the curvature of spacetime flattens out. Is that driving expansion?

So a technology that can tap into that energy is first working against the extra expansion of the universe before it would begin to cause a contraction. Right?

Been done in literature. The absurdist outcome is that capable civilizations eventually expand at nearly the speed of light in order not to be overcome by equivalent civilizations. Lesser civilizations are therefore unaware that they are even threatened until the expansion wave hits them.

..and by a simple optical illusion, the advancing wave could appear to approach at many times the speed of light. This idea does not need inertialess drives or black holes. They could just be fleeing exponential expansion of the races behind them. They might not even be fleeing any great catastrophe. It could simply be that if you fall behind, you are now part of the colonised interior and will never catch up to contribute to the genes and memes of the edge. Natural selection will populate the edge with the genes and memes that keep you on the edge, just as if the alternative was death.

I have wondered if the same thing could happen on earth. What if people leave and find a utopia in space, but never return due to becoming adapted to lower gravity, or just because nothing on earth appeals to you anymore.

People on earth could look up and have all the evidence they need for how wonderful the alternative is, but natural selection would select for philosophies that teach people to ignore that evidence and choose to remain on earth.